Electronic Journal of Differential Equations, Vol. 2008(2008), No. 126, pp. 1–22. ISSN: 1072-6691. URL: http://ejde.math.txstate.edu or http://ejde.math.unt.edu ftp ejde.math.txstate.edu (login: ftp) CONSTRUCTION OF ALMOST PERIODIC SEQUENCES WITH GIVEN PROPERTIES MICHAL VESELY´ Abstract. We define almost periodic sequences with values in a pseudometric space X and we modify the Bochner definition of almost periodicity so that it remains equivalent with the Bohr definition. Then, we present one (easily modifiable) method for constructing almost periodic sequences in X . Using this method, we find almost periodic homogeneous linear difference systems that do not have any non-trivial almost periodic solution. We treat this problem in a general setting where we suppose that entries of matrices in linear systems belong to a ring with a unit. 1. Introduction First of all we mention the article [9] by Fan which considers almost periodic sequences of elements of a metric space and the article [22] by Tornehave about almost periodic functions of the real variable with values in a metric space. In these papers, it is shown that many theorems that are valid for complex valued sequences and functions are no longer true. For continuous functions, it was observed that the important property is the local connection by arcs of the space of values and also its completeness. However, we will not use their results or other theorems and we will define the notion of the almost periodicity of sequences in pseudometric spaces without any conditions, i.e., the definition is similar to the classical definition of Bohr, the modulus being replaced by the distance. We also refer to [31] (or [28]). We add that the concept of almost periodic functions of several variables with respect to Hausdorff metrics can be found in [19] which is an extension of [8]. In Banach spaces, a sequence {ϕk}k∈Z is almost periodic if and only if any se- quence of translates of {ϕk} has a subsequence which converges and its convergence is uniform with respect to k in the sense of the norm. In 1933, the continuous case of the previous result was proved by Bochner in [4], where the fundamental theo- rems of the theory of almost periodic functions with values in a Banach space are proved too – see, e.g., [2], [3, pp. 3–25] or [15], where the theorems have been re- demonstrated by the methods of the functional analysis. We add that the discrete 2000 Mathematics Subject Classification. 39A10, 42A75. Key words and phrases. keywords Almost periodic sequences; almost periodic solutions to difference systems. c 2008 Texas State University - San Marcos. Submitted October 9, 2007. Published September 10, 2008. Supported by grant 201/07/0145 of the Czech Grant Agency. 1 2 M. VESELY´ EJDE-2008/126 version of this result can be proved similarly as in [4]. We also mention directly the papers [26] and [17]. In pseudometric spaces, the above result is not generally true. Nevertheless, by a modification of the Bochner proof of this result, we will prove that a necessary and sufficient condition for a sequence {ϕk}k∈Z to be almost periodic is that any sequence of translates of {ϕk} has a subsequence satisfying the Cauchy condition, uniformly with respect to k. The paper is organized as follows. The next section presents the definition of almost periodic sequences in a pseudometric space, the above necessary and suffi- cient condition for the almost periodicity of a sequence {ϕk}k∈Z, and some basic properties of almost periodic sequences in pseudometric spaces (see also, e.g., [16]). In Section 3, we show the way one can construct almost periodic sequences in pseudometric spaces. We present it in the below given theorems. In Theorem 3.1, we consider almost periodic sequences for k ∈ N0; in Theorem 3.3 and Corolla- ry 3.4, sequences for k ∈ Z obtained from almost periodic sequences for k ∈ N0; and, in Theorems 3.5 and 3.6, sequences for k ∈ Z. We remark that our process is comprehensible and easily modifiable. We add that methods of generating almost periodic sequences are mentioned also in [16, Section 4]. Then, in Section 4, we use results from the second and the third section of this paper to obtain a theorem which will play important role in the article [23], where it is proved that the almost periodic homogeneous linear difference systems which do not have any nonzero almost periodic solutions form a dense subset of the set of all considered systems. Using our method, one can get generalizations of statements from [21] and [24], where unitary (and orthogonal) systems are studied (see also [25]). We will analyse systems of the form xk+1 = Ak · xk, k ∈ Z (or k ∈ N0), where {Ak} is almost periodic. We want to prove that there exists a system of the above form which does not have an almost periodic solution other than the trivial one. (See Theorem 4.7.) A closer examination of the methods used in constructions reveals that the problem can be treated in possibly the most general setting: (1) almost periodic sequences attain values in a pseudometric space; (2) the entries of almost periodic matrices are elements of an infinite ring with a unit. We note that many theorems about the existence of almost periodic solutions of almost periodic difference systems of general forms are published in [11, 12, 18, 28, 29, 31] and several these existence theorems are proved there in terms of discrete Lyapunov functions. Here, we can also refer to the monograph [27] and [32, Theorems 3.6, 3.7, 3.8]. We add that the existence of an almost periodic homogeneous linear differential system, which has nontrivial bounded solutions and, at the same time, all the systems from some neighbourhood of it have no nontrivial almost periodic solutions, is proved in [20]. + As usual, R denotes the real line, R0 the set of all nonnegative reals, C the complex plane, Z denotes the set of integers, N the set of natural numbers, and N0 the set of positive integers including the zero 0. + Let X 6= ∅ be an arbitrary set and let d : X × X → R0 have these properties: (i) d(x, x) = 0 for all x ∈ X , EJDE-2008/126 CONSTRUCTION OF ALMOST PERIODIC SEQUENCES 3 (ii) d(x, y) = d(y, x) for all x, y ∈ X , (iii) d(x, y) ≤ d(x, z) + d(z, y) for all x, y, z ∈ X . We say that d is a pseudometric on X and (X , d) a pseudometric space. For given ε > 0, x ∈ X , in the same way as in metric spaces, we define the ε-neighbourhood of x in X as the set {y ∈ X ; d(x, y) < ε}. It will be denoted by Oε(x). All sequences, which we will consider, will be subsets of X . The scalar (and vector) valued sequences will be denoted by the lower-case letters, the matrix valued sequences by the capital letters (X is a set of matrices in this case), and each one of the scalar and matrix valued sequences by the symbols {ϕk}, {ψk}, {χk}. 2. Almost periodic sequences in pseudometric spaces Now we introduce a “natural” generalization of the almost periodicity. We re- mark that our approach is very general and that the theory of almost periodic se- quences presented here does not distinguish between x ∈ X and y ∈ X if d(x, y) = 0. Definition 2.1. A sequence {ϕk} is called almost periodic if, for any ε > 0, there exists a positive integer p(ε) such that any set consisting of p(ε) consecutive integers (nonnegative integers if k ∈ N0) contains at least one integer l with the property that d(ϕk+l, ϕk) < ε, k ∈ Z (or k ∈ N0). In the above definition, l is called an ε-translation number of {ϕk}. Consider again ε > 0. Henceforward, the set of all ε-translation numbers of a sequence {ϕk} will be denoted by T({ϕk}, ε). Remark 2.2. If X is a Banach space (d(x, y) is given by || x−y ||), then a necessary and sufficient condition for a sequence {ϕk}k∈Z to be almost periodic is it to be normal; i.e., {ϕk} is almost periodic if and only if any sequence of translates of {ϕk} has a subsequence, uniformly convergent for k ∈ Z in the sense of the norm. This statement and the below given Theorem 2.3 are not valid if {ϕk} is defined for k ∈ N0 and if we consider only translates to the right – see the example X = R, ϕ0 = 1, and ϕk = 0, k ∈ N. But, if we consider translates to the left, then both of results are valid for k ∈ N0 too. It is seen that the above result is no longer valid if the space of values fails to be complete. Especially, in a pseudometric space (X , d), it is not generally true that a sequence {ϕk}k∈Z is almost periodic if and only if it is normal. Nevertheless, applying the methods from any one of the proofs of the results [6, Theorem 1.10, p. 16], [10, Theorem 1.14, pp. 9–10], and [3, Statement (ζ), pp. 8–9], one can easily prove that every normal sequence {ϕk}k∈Z is almost periodic. Further, we can prove the next theorem which we will need later. We add that its proof is a modification of the proof of [6, Theorem 1.26, pp. 45–46]. Theorem 2.3. Let {ϕk}k∈Z be given.